1. Field of the Invention
The present invention relates to automatic focus control technique of an optical microscope and an electron microscope; for example, the present invention is utilized to a system measuring critical dimension of minute pattern, that is, a critical dimension SEM and so on.
2. Description of the Related Background Art
In semiconductor production lines, in order to measure and control the critical dimension of the minute pattern, a critical dimension SEM using electron beam is utilized. Such a kind of the SEM is now completely automated. In case of measuring the critical dimension of the pattern, at each measuring point in a wafer, the process for automatic focus adjustment is performed, and then the measurement of the pattern's critical dimension is performed. The reason why the process for automatic focus adjustment is performed at each measuring point is a warp's amount of the wafers is normally larger than the depth of focus of the SEM, which is about 1 .mu.m.
In case it takes long time to perform automatic focus adjustment process, it is unable to improve throughput for the critical dimension's measurement. Accordingly, the method which shortens the time necessary to perform the process for automatic focus adjustment by providing an optical Z-sensor mechanism and so on is also being examined.
However, in order to perform the ultimate focus control, it is necessary to control an object lens composed of an electromagnetic coil and so on. It takes the processing time of about 10 seconds per a focus control. Accordingly, in case of performing the process for automatic focus adjustment, the time of (10 seconds * the number of the measuring point) is required.
Thus, in case the process for automatic focus adjustment is performed at each measuring point, it is unable to improve the throughput of the critical dimension's measurement. Particularly, if the wafer's diameter becomes larger in the future, the number of the measuring points also increases. Therefore, the measuring time per a wafer becomes longer.
Because the electron microscope irradiates the electron beam on the wafer, the surface of the wafer temporarily takes a charge. Consequently, the impurity of the floating positive ion is deposited on the surface of the wafer to contaminate the surface of the wafer. Accordingly, it is desirable to decrease the amount of irradiation of the electron beam as possible. Because the electron beam has to be irradiated even for the purpose of performing the process for automatic focus adjustment, the surface on the wafer is terribly contaminated when the process for automatic focus adjustment is performed.